The high electron mobility transistor (HEMT) is a type of field effect transistor (FET) in which a hetero-junction between a channel layer and a barrier layer whose electron affinity is smaller than that of the channel layer. A two-dimensional electron gas (2DEG) forms in the channel layer of a group III-V HEMT device due to the mismatch in polarization field at the channel-barrier layer interface. The 2DEG has a high electron mobility that facilitates high-speed switching during device operation. In typical depletion-mode HEMT devices (also known as normally-on devices), a negatively-biased voltage may be applied to the gate electrode to deplete the 2DEG and thereby turn off the device. A group III-V HEMT device is one made of materials in column III of the periodic table, such as aluminum (Al), gallium (Ga), and indium (In), and materials in column V of the periodic table, such as nitrogen (N), phosphorus (P), and arsenic (As).
Group III-Nitride HEMT devices are especially suited for power electronics end-applications operating under voltage and current conditions that cannot be achieved with conventional silicon (Si)-based transistor devices. In order to suppress leakage current and to sustain high voltages without breaking down, group III-Nitride HEMT devices typical employ a highly resistive layer underlying the channel layer. The highly resistive layer commonly comprises a layer of gallium nitride (GaN) doped with carbon (C) or iron (Fe), with C doping being the most typical approach. However, doping GaN with C or Fe introduces defects in the material, which results in an increase in the on-resistance of the HEMT device when stressed at a high voltage. This changing on-resistance is known as current collapse, and it is one problem hindering the widespread adoption of group III-Nitride HEMT devices today.
FIG. 1 shows a plot of the current collapse ratio of an HEMT device containing a highly resistive C-doped GaN layer as a function of the thickness of the highly resistive layer. The current collapse ratio is the ratio of the measured on-resistance of the HEMT device after applying a high voltage compared to the measured on-resistance of the HEMT device before applying a high voltage. 200V is applied to the gate of the HEMT device of FIG. 1 to measure the current collapse ratio. As shown in FIG. 1, the current collapse ratio in the HEMT device is directly proportional to the amount of C-doped GaN incorporated into the HEMT device.
When there is no C-doped GaN in the HEMT device, the current collapse ratio is about 1, or in other words, the on-resistance of the HEMT device changed little, if at all, after 200V was applied to the gate of the HEMT device. Conversely, when the HEMT device has a C-doped GaN layer having a thickness of 3 μm, the measured current collapse ratio increases to about 1.2 to 1.3, or a 20% to 30% increase in the on-resistance of the HEMT device after 200V was applied to the gate.
There is, therefore, an unmet demand for HEMT devices that suppress the current collapse caused by C doping in the highly resistive layer while maintaining low leakage current and high breakdown voltage characteristics.